Scavengeless development system having toner deposited on a doner roller from a toner mover

ABSTRACT

An apparatus which develops an electrostatic latent image recorded on a photoconductive member with toner. Fluidized toner is transported from one end of the developer housing to the other end. An electrical bias is applied between the toner mover and the donor roller so as to attract toner to the donor roller. A rotating rod resiliently urged into contact with the donor roller charges and meters the toner on the donor roller. Electrically biased wires in the development zone between the photoconductive member and the donor roller detach toners from the donor roll so as to form a toner powder cloud in the development zone. Detached toner from the toner powder cloud develops the latent image.

This invention relates generally to an electrophotographic printingmachine, and more particularly concerns a single component developmentsystem.

Generally, the process of electrophotographic printing includes charginga photoconductive member to a substantially uniform potential so as tosensitize the surface thereof. The charged portion of thephotoconductive surface is exposed to a light image of an originaldocument being reproduced. This records an electrostatic latent image onthe photoconductive surface. After the electrostatic latent image isrecorded on the photoconductive surface, the latent image is developedby bringing a developer material into contact therewith. Two componentand single component developer materials are commonly used. A typicaltwo component developer material comprises magnetic carrier granuleshaving toner particles adhering triboelectrically thereto. A singlecomponent developer material typically comprises toner particles. Tonerparticles are attracted to the latent image forming a toner powder imageon the photoconductive surface. The toner powder image is subsequentlytransferred to a copy sheet. Finally, the toner powder image is heatedto permanently fuse it to the copy sheet in image configuration.

Single component development systems use a donor roll for transportingcharged toner to the development nip defined by the donor roll andphotoconductive member. The toner is developed on the latent imagerecorded on the photoconductive member by a combination of mechanicaland/or electrical forces. One type of single component development isscavengeless development. A scavengeless development system uses a donorroll with a plurality of electrode wires closely spaced therefrom in thedevelopment zone. An AC voltage is applied to the wires forming a tonercloud in the development zone. The electrostatic fields generated by thelatent image attract toner from the toner cloud to develop the latentimage. A large continuous supply of toner particles must be available tobe capable of copying large numbers of original documents producingmultiple copies of the same original document. This is necessary inorder to insure that the printing machine is not shut down afterrelatively short operating intervals due to the lack of toner particles.This is achieved by storing a supply of toner particles in a tonercontainer and dispensing additional toner particles into one end of thedeveloper housing chamber. The toner particles are then transportedacross the chamber of the developer housing and advanced to the donorroller. However, it has been found that it is frequently difficult toload the donor roller with a sufficient quantity of toner particlesuniformly along the length thereof. This has resulted in image defectswhich degrade the quality of the copy. The following disclosures may berelevant to various aspects of the present invention:

U.S. Pat. No. 4,833,504, Patentee: Parker et al. Issued: May 23, 1989

U.S. Pat. No. 4,868,600, Patentee: Hays et al. Issued: Sep. 19, 1989

U.S. Pat. No. 4,868,611, Patentee: Germain Issued: Sep. 19, 1989

U.S. Pat. No. 4,876,575, Patentee: Hays Issued: Oct. 24, 1989

U.S. Pat. No. 4,990,958, Patentee: Brewington et al. Issued: Feb. 5,1991

Co-pending U.S. patent application Ser. No. 07/428,726, Applicant:Brewington et al. Filed: Oct. 30, 1989

Co-pending U.S. patent application Ser. No. 07/537,660, Applicant:Brewington et al. Filed: Jun. 14, 1990

The relevant portions of the foregoing disclosures may be brieflysummarized as follows:

U.S. Pat. No. 4,833,504 and U.S. Pat. No. 4,868,611 disclose a singlepass highlight color printer having a scavengeless developer unitcontaining a plurality of developer rolls. The developer rolls haverotating non-magnetic cylinders or shells with roughened surfaces andstationary magnets.

U.S. Pat. No. 4,868,600 describes a scavengeless development apparatusin which toner detachment from a donor roll and generation of acontrolled powder cloud is obtained by AC electrically biased electrodewires.

U.S. Pat. No. 4,876,575 discloses a donor roll having a rotatingmetering and charging rod forming a toner metering and charging zonethrough which toner is moved to simultaneously charge and meter thetoner particles. The rod is supported by a distributed bearing attachedto a compliant blade. A toner cleaning blade held against the rod servesas a toner seal. The rod is electrically biased.

U.S. Pat. No. 4,990,958 describes a scavengeless development apparatusin which toner detachment from a donor roll and generation of acontrolled powder cloud is obtained by AC electrically biased electrodewires. A reload member supported in rubbing contact with an electricallybiased toner mover effects reloading of the donor roll with toner. Thetoner mover serves to transport toner from a remote supply of toner toan area in the opposite the donor roll.

Co-pending U.S. patent application Ser. No. 07/428,726 and co-pendingU.S. patent application Ser. No. 07/537,660 describe a developmentsystem having a hollow tube having holes therein which fluidizes andmoves toner particles from one end of a developer housing to the otherend thereof. The tube is electrically biased so that toner particles areattracted from the tube to a donor roller. A charging blade ismaintained in contact with the donor roll to charge the toner layer onthe donor roller.

In accordance with one aspect of the present invention, there isprovided an apparatus for developing a latent image recorded on asurface. The apparatus includes a housing defining a chamber storing asupply of marking particles therein. Means, disposed in the chamber ofthe housing, fluidize the marking particles. A donor member, spaced fromthe surface, is adapted to transport marking particles to a developmentzone adjacent the surface. Means apply an electrical bias between thefluidizing means and the donor member so as to attract fluidized markingparticles to the donor member. An electrode member is positioned in thedevelopment zone between the surface and the donor member. The electrodemember is electrically biased to detach marking particles from the donormember so as to form a cloud of marking particles in the developmentzone. Detached marking particles from the cloud of marking particlesdevelop the latent image.

Pursuant to another aspect of the present invention, there is providedan electrophotographic printing machine of the type in which anelectrostatic latent image recorded on a photoconductive member isdeveloped to form a visible image thereof. The improvement includes ahousing defining a chamber storing a supply of toner particles therein.Means, disposed in the chamber of said housing, fluidize the tonerparticles. A donor member, spaced from the surface, is adapted totransport toner particles to a development zone adjacent thephotoconductive member. Means apply an electrical bias between thefluidizing means and the donor member so as to attract fluidized markingparticles to the donor member. An electrode member is positioned in thedevelopment zone between the photoconductive member and the donormember. The electrode member is electrically biased to detach tonerparticles from the donor member so as to form a cloud of toner particlesin the development zone. Detached toner particles from the cloud oftoner particles develop the latent image.

Other features of the present invention will become apparent as thefollowing description proceeds and upon reference to the drawings, inwhich:

FIG. 1 is a schematic elevational view of an illustrativeelectrophotographic printing machine incorporating a developmentapparatus having the features of the present invention therein;

FIG. 2 is an elevational view, partially in section, showing thedevelopment apparatus used in the FIG. 1 printing machine; and

FIG. 3 is a schematic side elevational view of the FIG. 2 developmentapparatus.

While the present invention will be described in connection with apreferred embodiment thereof, it will be understood that it is notintended to limit the invention to that embodiment. On the contrary, itis intended to cover all alternatives, modifications, and equivalents asmay be included within the spirit and scope of the invention as definedby the appended claims.

Inasmuch as the art of electrophotographic printing is well known, thevarious processing stations employed in the FIG. 1 printing machine willbe shown hereinafter schematically and their operation described brieflywith reference thereto.

Referring initially to FIG. 1, there is shown an illustrativeelectrophotographic printing machine incorporating the developmentapparatus of the present invention therein. The electrophotographicprinting machine employs a belt 10 having a photoconductive surface 12deposited on a conductive substrate 14. Preferably, photoconductivesurface 12 is made from a selenium alloy. Conductive substrate 14 ismade preferably from an aluminum alloy which is electrically grounded.One skilled in the art will appreciate that any suitable photoconductivebelt may be used. Belt 10 moves in the direction of arrow 16 to advancesuccessive portions of photoconductive surface 12 sequentially throughthe various processing stations disposed about the path of movementthereof. Belt 10 is entrained about stripping roller 18, tensioningroller 20 and drive roller 22. Drive roller 22 is mounted rotatably inengagement with belt 10. Motor 24 rotates roller 22 to advance belt 10in the direction of arrow 16. Roller 22 is coupled to motor 24 bysuitable means, such as a drive belt. Belt 10 is maintained in tensionby a pair of springs (not shown) resiliently urging tensioning roller 20against belt 10 with the desired spring force. Stripping roller 18 andtensioning roller 20 are mounted to rotate freely.

Initially, a portion of belt 10 passes through charging station A. Atcharging station A, a corona generating device, indicated generally bythe reference numeral 26 charges photoconductive surface 12 to arelatively high, substantially uniform potential. High voltage powersupply 28 is coupled to corona generating device 26. Excitation of powersupply 28 causes corona generating device 26 to charge photoconductivesurface 12 of belt 10. After photoconductive surface 12 of belt 10 ischarged, the charged portion thereof is advanced through exposurestation B.

At exposure station B, an original document 30 is placed face down upona transparent platen 32. Lamps 34 flash light rays onto originaldocument 30. The light rays reflected from original document 30 aretransmitted through lens 36 to form a light image thereof. Lens 36focuses this light image onto the charged portion of photoconductivesurface 12 to selectively dissipate the charge thereon. This records anelectrostatic latent image on photoconductive surface 12 whichcorresponds to the informational areas contained within originaldocument 30.

After the electrostatic latent image has been recorded onphotoconductive surface 12, belt 10 advances the latent image todevelopment station C. At development station C, a developer unit,indicated generally by the reference numeral 38, develops the latentimage recorded on the photoconductive surface. Preferably, developerunit 38 includes donor roller 40 and electrode wires 42. Electrode wires42 are electrically biased relative to donor roll 40 to detach tonertherefrom so as to form a toner powder cloud in the gap between thedonor roller and the photoconductive surface. The latent image attractstoner particles from the toner powder cloud forming a toner powder imagethereon. Donor roller 40 is mounted, at least partially, in the chamberof developer housing 44. The chamber in developer housing 44 stores asupply of developer material. The developer material is a singlecomponent developer material of toner particles. A toner mover disposedinteriorly of the chamber of housing 44 conveys the toner from one endof developer housing 44 to the other end thereof. As the toner traversesthe developer housing, toner is attracted from the toner mover to thedonor roller. The toner mover is electrically biased relative to thedonor roller so that the toner particles are attracted from the tonermover to the donor roller. Developer unit 38 will be discussedhereinafter, in greater detail, with reference to FIGS. 2 and 3.

With continued reference to FIG. 1, after the electrostatic latent imageis developed, belt 10 advances the toner powder image to transferstation D. A copy sheet 48 is advanced to transfer station D by sheetfeeding apparatus 50. Preferably, sheet feeding apparatus 50 includes afeed roll 52 contacting the uppermost sheet of stack 54. Feed roll 52rotates to advance the uppermost sheet from stack 54 into chute 56.Chute 56 directs the advancing sheet of support material into contactwith photoconductive surface 12 of belt 10 in a timed sequence so thatthe toner powder image developed thereon contacts the advancing sheet attransfer station D. Transfer station D includes a corona generatingdevice 58 which sprays ions onto the back side of sheet 48. Thisattracts the toner powder image from photoconductive surface 12 to sheet48. After transfer, sheet 48 continues to move in the direction of arrow60 onto a conveyor (not shown) which advances sheet 48 to fusing stationE.

Fusing station E includes a fuser assembly, indicated generally by thereference numeral 62, which permanently affixes the transferred powderimage to sheet 48. Fuser assembly 62 includes a heated fuser roller 64and a back-up roller 66. Sheet 48 passes between fuser roller 64 andback-up roller 66 with the toner powder image contacting fuser roller64. In this manner, the toner powder image is permanently affixed tosheet 48. After fusing, sheet 48 advances through chute 70 to catch tray72 for subsequent removal from the printing machine by the operator.

After the copy sheet is separated from photoconductive surface 12 ofbelt 10, the residual toner particles adhering to photoconductivesurface 12 are removed therefrom at cleaning station F. Cleaning stationF includes a rotatably mounted fibrous brush 74 in contact withphotoconductive surface 12. The particles are cleaned fromphotoconductive surface 12 by the rotation of brush 74 in contacttherewith. Subsequent to cleaning, a discharge lamp (not shown) floodsphotoconductive surface 12 with light to dissipate any residualelectrostatic charge remaining thereon prior to the charging thereof forthe next successive imaging cycle.

It is believed that the foregoing description is sufficient for purposesof the present application to illustrate the general operation of anelectrophotographic printing machine incorporating the developmentapparatus of the present invention therein.

Referring now to FIGS. 2 and 3, there is shown developer unit 38 ingreater detail. As shown thereat, developer unit 38 includes a housing44 defining a chamber 76 for storing a supply of developer materialtherein. Donor roller 40, electrode wires 42 and toner mover 46 aremounted in chamber 76 of housing 44. The donor roller can be rotated ineither the `with` or `against` direction relative to the direction ofmotion of belt 10. In FIG. 2, donor roller 40 is shown rotating in thedirection of arrow 68, i.e. the against direction. Similarly, the tonermover 46 can be rotated in either the `with` or `against` directionrelative to the direction of motion of belt 10. In FIG. 2, toner mover46 is shown rotating in the direction of arrow 92 i.e. the againstdirection. A charging rod 78 is resiliently urged into engagement withdonor roller 40. Charging rod 78 rotates in the direction of arrow 80. ADC voltage source electrical biases the charging rod relative to thedonor roll. A leaf spring 82 supports charging rod 78. The leaf spring82 is mounted in chamber 76 of housing 44 with the free end rotatablysupporting charging rod 78. One skilled in the art will appreciate thatany suitable spring may be used to support charging rod 78 and toresiliently urge it into contact with donor roller 40. Leaf spring 82 ispreferably made from sheet steel. Charging rod 78 charges the tonerparticles adhering to donor roller 40 and regulates the thickness of thelayer of toner particles on donor roller 40. Preferably, charging rod 78is made from aluminum having a nickel coating of about 0.013 mm. Donorroller 40 is preferably made from aluminum having apolytetrafluoroethylene based resin of about 0.05 mm coated thereon.Teflon-S, a trademark of the DuPont Corporation is one such suitableresin. This coating acts to assist in charging the toner particlesadhering to the surface thereof.

Development system 38 also has electrode wires 42 which are disposed inthe space between the belt 10 and donor roller 40. A pair of electrodewires are shown extending in a direction substantially parallel to thelongitudinal axis of the donor roller. The electrode wires are made fromof one or more thin (i.e. 50 to 100μ in diameter) metal, e.g. tungsten,wires which are self-spaced from the donor roller by the thickness ofthe toner on the donor roller.

As illustrated in FIG. 2, an alternating electrical bias is applied tothe electrode wires by an AC voltage source 84. The applied ACestablishes an alternating electrostatic field between the wires and thedonor roller which is effective in detaching toner from the surface ofthe donor roller and forming a toner cloud about the wires, the heightof the cloud being such as not to be substantially in contact with thebelt 10. The magnitude of the AC voltage is relatively low and is in theorder of 200 to 600 volts peak at a frequency ranging from about 3 KHzto about 20 KHz. A DC bias supply 86 which applies approximately 300volts to donor roller 40 establishes an electrostatic field betweenphotoconductive surface 12 of belt 10 and donor roller 40 for attractingthe detached toner particles from the cloud surrounding the wires to thelatent image recorded on the photoconductive surface. At a maximumspacing of about 0.025 mm between the electrode wires and donor roller,an applied AC voltage of 200 to 600 volts produces a relatively largeelectrostatic field without risk of air breakdown. The use of adielectric coating on either the electrode wires or donor roller helpsto prevent shorting of the applied AC voltage. A DC bias supply 88 whichapplies approximately 500 to 1000 volts to toner mover 46 establishes anelectrostatic field between toner mover 46 and donor roller 40 so thatan electrostatic field is established between the donor roller and thetoner mover which causes toner particles to be attracted from themagnetic roller to the donor roller. Toner mover 46 fluidizes the tonerparticles. The fluidized toner particles seek their own level under theinfluence of the gravity. Inasmuch as new toner particles are beingdischarged from container 90, located at one end of housing 44 (FIG. 3),into one end of the chamber 78, the force exerted on the fluidized tonerparticles by the new toner particles being added at that end moves thefluidized toner particles from that end of housing 44 to the other endthereof. Toner mover 46 is an elongated member located in chamber 78closely adjacent to an arcuate portion 94 of housing 44. Arcuate portion94 is closely adjacent to elongated member 46 and wraps about a portionthereof. There is a relatively small gap or space between arcuateportion 94 and a portion of elongated member 46. New toner particles aredischarged into one end of chamber 78 from container 90. The elongatedmember is shorter than the donor roll to decrease toner loading on thedonor roll at the ends thereof. This alleviates toner head pressure onthe end seals. For a design using end feeding of toner, only thenon-feed end has the elongated member shorter than the donor roll. Thedonor roll has detone pads in rubbing contact with the surface thereofto decrease or eliminate the toner loading on the donor roll at the endsthereof. As elongated member 46 rotates in the direction of arrow 92,toner particles are fluidized. A motor (not shown) rotates elongatedmember 46 at an angular velocity ranging from about 200 to about 600revolutions per minute with the preferred set point being about 400revolutions per minute. The force exerted on the fluidized tonerparticles by the new particles being discharged into chamber 76 advancesthe fluidized toner particles from the end of the chamber in which thenew toner particles have been discharged to the other end thereof. Thefluidized toner particles being moved are attracted to donor roller 40.Elongated member 46 is made from an electrically conductive material,such as aluminum. Voltage source 88 is electrically connected toelongated member 46. Elongated member 46 is spaced from donor roller 40to define a gap therebetween. This gap is preferably about 1.0 mm. Donorroller 40 rotates in the direction of arrow 68 to move the tonerparticles attracted thereto into contact with the electrostatic latentimage recorded on photoconductive surface 12 of belt 10. As donor roller40 rotates in the direction of arrow 68, charging rod 78 is resilientlyurged into contact with donor roller 40. Charging rod 78 is maintainedin contact with donor roller 74 at a nominal nip force ranging fromabout 25 grams per centimeter to about 100 grams per centimeter. Thetoner particle layer adhering to donor roller 74 is charged to a maximumof 40 microcoulombs/gram with the toner mass adhering thereto rangingfrom about 0.1 milligrams/centimeter² to about 2 milligrams/centimeter²of roll surface. It is thus seen that elongated member 46 continuallyfluidizes these toner particles. These fluidized toner particles areattracted from elongated member 46 to donor roller 40. Donor roller 40transports these toner particles in the direction of arrow 68. The tonerparticles adhering to donor roller 40 have a net charge due toelectrostatic selection from the supply of elongated member 46, and arefurther charged by charging rod 78 prior to advancing into contact withthe electrostatic latent image recorded on photoconductive surface 12.These toner particles are attracted to the electrostatic latent image toform a toner powder image on photoconductive surface 12 of belt 10.

As shown in FIG. 2, elongated member 46 includes a hollow rod or tubehaving four equally spaced rows of apertures or holes therein. Each rowof holes is spaced about the periphery of rod by about 90°. Each hole ineach row is spaced from the next adjacent hole. The holes are equallyspaced from one another. In this way, as the tube rotates, the tonerparticles travel through the center of the tube and out through thevarious holes so as to be fluidized. The fluidized toner particles areadvanced from one end of the chamber of the developer housing to theother end thereof by the back pressure exerted by the head of fresh ornew toner particles being discharged into the chamber from the tonerstorage container 90 (FIG. 3). Alternatively, elongated member 46 may bea rod having a cylindrical member mounted thereon. The cylindricalmember has a plurality of spaced saw tooth shaped paddles extendingoutwardly therefrom. As elongated member 46 rotates, the paddles agitateand fluidize the toner particles. The toner particles fly off the tipsof the saw tooth shaped paddles so as to be fluidized. The pressure orforce exerted on the fluidized toner particles by the new tonerparticles being discharged from toner container 90 (FIG. 3) moves thefluidized toner particles from one end of the chamber 76 (FIG. 2) ofhousing 44 (FIG. 2) to the other end thereof.

In recapitulation, it is evident that the development apparatus of thepresent invention includes electrode wires positioned closely adjacentthe exterior surface of a donor roller and in the gap defining thedevelopment zone between the donor roller and the photoconductive belt.A toner mover fluidizes the toner particles in the chamber of thedeveloper housing. Toner particles being added to the chamber at one endthereof exert a pressure on the fluidized toner particles to move thetoner particles from one end of the developer housing chamber to theother end thereof. The toner mover and the donor roller are electricallybiased relative to one another so that as the toner particles move fromone end of the developer housing to the other end, they are attracted tothe donor roller. A rotating charging rod resiliently urged into contactwith the donor roller charges the toner particles and regulates thequantity of toner particles on the donor roller. An AC voltage isapplied to the electrode wires to detach toner particles from the donorroller so that a toner powder cloud is formed in the development zone.Detached toner particles from the toner powder cloud are attracted tothe latent image recorded on the photoconductive belt to develop thelatent image.

It is, therefore, apparent that there has been provided in accordancewith the present invention, a development system that fully satisfiesthe aims and advantages hereinbefore set forth. While this invention hasbeen described in conjunction with a specific embodiment thereof, it isevident that many alternatives, modifications, and variations will beapparent to those skilled in the art. Accordingly, it is intended toembrace all such alternatives, modifications and variations that fallwithin the spirit and broad scope of the appended claims.

We claim:
 1. An electrophotographic printing machine of the type inwhich an electrostatic latent image recorded on a photoconductive memberis developed to form a visible image thereof, wherein the improvementincludes:a housing defining a chamber storing a supply of tonerparticles therein; a doner roller spaced from the photoconductive memberand adapted to transport toner particles to a development zone adjacentthe photoconductive member; a rotatably mounted elongated memberdisposed in the chamber of said housing in the region of said donerroller; means for applying an electrical bias between said elongatedmember and said doner roller so as to attract fluidized toner particlesto said doner roller; means for charging and metering the quantity oftoner particles being advanced by said doner roller to the electrostaticlatent image recorded on the photoconductive member, said charging andmetering means comprising a rotating rod, and means for resilientlyurging said rod closely adjacent to said doner roller; a plurality ofsmall diameter wires positioned in the development zone between thephotoconductive member and the said doner roller; and means forelectrically biasing said electrode wires to detach toner particles fromsaid doner roller so as to form a cloud of toner particles in thedevelopment zone with detached toner particles from the cloud of tonerparticles developing the latent image.
 2. A printing machine accordingto claim 1, further including means for discharging additional markingparticles into the chamber of said housing with the marking particlesbeing discharged into the chamber exerting a pressure on the fluidizedmarking particles to move the fluidized marking particles from one endof said housing to the other end thereof.